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Acta Physica Polonica A

Singlet-Triplet Dephasing in Asymmetric Quantum Dot Molecules

Łukasz Marcinowski, Katarzyna Roszak, Paweł Machnikowski

Published Nov 1, 2009• 1 citation
Publisher

Abstract

We discuss pure dephasing of singlet-triplet superpositions in two-electron double quantum dots due to elastic phonon scattering.We generalize our previous results to a system built of two non-identical dots.We show that the asymmetry must be very strong in order to considerably affect the dephasing rate.
DOI
10.12693/aphyspola.116.874

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We show that singlet-triplet superpositions of two-electron spin states in a double quantum dot undergo a phonon-induced pure dephasing which relies only on the tunnel coupling between the dots and on the Pauli-exclusion principle. As such, this dephasing process is independent of spin-orbit coupling or hyperfine interactions. The physical mechanism behind the dephasing is elastic phonon scattering, which persists to much lower temperatures than real phonon-induced transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs gate-defined double quantum dot yield microsecond dephasing times at sub-Kelvin temperatures, which is consistent with experimental observations.
Phonon-induced dephasing of two-electron spin states in two vertically stacked self-assembled GaAs/InGaAs quantum dots is studied.A pure dephasing process due to elastic phonon scattering is found to dominate at low temperatures.This process is independent of the spin-orbit coupling and does not require the presence of a magnetic field.It relies on interdot tunneling and the Pauli principle, which make the double quantum dot gate for spin based quantum computing possible, and therefore cannot be avoided.
We study electron-phonon interaction induced decoherence between two-electron singlet and triplet states in a semiconductor double quantum dot using a spin-boson model. We investigate the onset and time evolution of this dephasing, and study its dependence on quantum dot parameters such as dot size and double dot separations, as well as the host materials (GaAs and Si). At the short time limit, electron-phonon interaction only causes an incomplete initial Gaussian decay of the off-diagonal density matrix element in the singlet-triplet Hilbert space; a complete long-time exponential decay due to phonon relaxation would eventually dominate over two-spin decoherence. We analyze two-spin decoherence in both symmetric and biased double quantum dots, identifying their difference in electron-phonon coupling and the relevant consequences.
The optical initialization and readout of an electron spin in a quantum dot involves often the lowest excited states of two electrons in a negative trion. The dynamics involves mixing of electron triplets and excited singlets. Typically, the mixing mechanism invoked has been electron‐hole exchange in the presence of dot lateral asymmetries, but this has not been always consistent with the observed dephasing rates. We describe here a more general and stronger singlet‐triplet mixing mechanism that does not require dot asymmetry.
We investigate theoretically the hyperfine-induced dephasing of two-electron-spin states in a double quantum dot with a finite singlet-triplet splitting JJJ. In particular, we derive an effective pure dephasing Hamiltonian, which is valid when the hyperfine-induced mixing is suppressed due to the relatively large JJJ and the external magnetic field. Using both a quantum theory based on resummation of ring diagrams and semiclassical methods, we identify the dominant dephasing processes in regimes defined by values of the external magnetic field, the singlet-triplet splitting, and inhomogeneity in the total effective magnetic field. We address both free induction and Hahn echo decay of superposition of singlet and unpolarized triplet states (both cases are relevant for singlet-triplet qubits realized in double quantum dots). We also study hyperfine-induced exchange gate errors for two single-spin qubits. Results for III-V semiconductors as well as silicon-based quantum dots are presented.